The present invention is directed to a fuel injector according to the definition of the species in the main claim.
Fuel injectors having a plurality of discharge orifices are known. They feature a plurality of discharge orifices, mostly designed as bore holes, downstream from a sealing seal formed by a valve needle and a valve seat surface. Fuel is discharged through these discharge orifices when the valve needle is lifted.
German Patent Application 198 27 219 A, for example, describes fuel injectors which have a spray orifice disk at the downstream end. Discharge orifices divided into several hole circles are arranged in this spray orifice disk. In order to form a certain discharge geometry, the discharge orifices are introduced in the spray orifice disk at different angles relative to the central axis of the fuel injector. Thus, for a flat spray orifice disk, the individual jets which are discharged from discharge orifices of the internal and external hole circles interfere with one another as they spread. In order to achieve sufficient jet deflection, the thickness of the spray orifice disk is so large that the flow length along the discharge orifice is large compared to the diameter of the discharge orifice.
Furthermore, a fuel injector is known from German Patent Application 198 04 463 A1 in which a plurality of discharge orifices are introduced in the valve seat body. The fuel injector is shaped conically outwardly in the area of the discharge orifices. The discharge orifices are introduced directly in the valve seat body and positioned on several hole circles, for example, downstream from the sealing seat.
Disadvantageous in the above-described fuel injectors are the thick-walled components in which the discharge orifices are to be introduced. These are required to withstand the high fuel pressure and combustion chamber pressure.
The radial dimensions of the discharge orifices cannot be selected to be as small as desired due to the thick-walled design, since limits are set by the possible aspect ratio as a result of the machining operations that can be used. The situation can be remedied by reducing the number of discharge orifices. This increases the radial dimensions of the individual discharge orifices while simultaneously preserving the total discharge cross-section. The result, however, is undesirable concentration gradients of the fuel mixture in the combustion chamber.
Conventional machining operations, such as drilling, for example, can, in fact, be employed to great workpiece depths; however, they increase the dimensional tolerances. The result is a greater tolerance for the flow rate. This makes optimizing the flow rate difficult, which ultimately results in higher consumption of the internal combustion engine and deterioration of the exhaust characteristics.
If the geometry of the fuel injector is not flat in the area of the discharge orifices, it is even more difficult to introduce the discharge orifices.
The fuel injector according to the present invention having the features of the main claim has the advantage over the related art that the flow-through screen is manufacturable from a thin membrane or a thin sheet of metal, for example. This allows very small discharge orifices to be introduced using cost-effective methods. For example, if the discharge orifices are punched into the flow-through screen, radial elongations in the area of the flow-through screen""s thickness may be easily implemented.
Another advantage attained by positioning the thin flow-through screen downstream from the valve seat body is that the flow-through screen does not have any mechanically supporting functions. The housing is terminated at the downstream end of the fuel injector by the valve seat body. Therefore, a plurality of small discharge orifices may be introduced in the flow-through screen, resulting in a distinct improvement in the conditioning of the discharged fuel, and the fuel forms a largely homogeneous mixture cloud.
The tolerances of the discharge orifices to be introduced may be kept tight using highly reproducible methods such as punching, for example. The resulting piece-to-piece scattering is small and facilitates the design of the fuel injector. Finally, in this manner, the fuel consumption of the engine may be reduced.
Advantageous refinements of the fuel injector according to the present invention having the characterizing features of the main claim are rendered possible by the measures delineated in the characterizing features of the subclaims.
Thus, for example, only a small number of recesses may be introduced in the valve seat body, which greatly facilitates machining. However, fuel is metered in through a plurality of small discharge orifices in the flow-through screen. This preserves the proper conditioning of the fuel spray, although only a small number of recesses must be introduced in the thick-walled valve seat body, which, in addition, may have coarse tolerances.
The valve seat body and the flow-through screen may have a dome-shaped geometry. This contributes to a low coking tendency, in addition to the possibility of introducing the discharge orifices in the thin flow-through screen perpendicularly, the flow-through screen only being given its final shape subsequently. This guarantees a perpendicular discharge of the fuel from the discharge orifices and prevents the flow-through screen from being wetted, which further reduces the danger of coking.
Furthermore, the design of the flow-through screen as a membrane is advantageous. Atomization may be supported by vibrations, which are easily induced in a thin membrane. Improved atomization also reduces the time required to vaporize the fuel. In particular, in direct injection engines, this enables injection with optimized consumption, since a retarded injection timing may be selected.
Due to the configuration of the inside of the valve seat body which matches that of the valve closing body, there is almost no dead volume. This prevents the undischarged fuel from being evaporated on the hot fuel injector after the end of the discharge operation, which would result in emission peaks. In addition, at the beginning of the following discharge operation the response time is reduced, since no volume has to be filled with fuel before the fuel pressure required for forming a fine fuel spray is applied to the discharge orifices.